|Publication number||US7979969 B2|
|Application number||US 12/710,341|
|Publication date||Jul 19, 2011|
|Priority date||Nov 17, 2008|
|Also published as||US20100210040|
|Publication number||12710341, 710341, US 7979969 B2, US 7979969B2, US-B2-7979969, US7979969 B2, US7979969B2|
|Inventors||Bulent M. Basol|
|Original Assignee||Solopower, Inc.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (17), Non-Patent Citations (1), Referenced by (8), Classifications (27), Legal Events (6)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This application claims priority to U.S. Provisional Patent Application No. 61/154,324 filed Feb. 20, 2009 and this application claims priority to and is a continuation in part of U.S. patent application Ser. No. 12/703,120 filed Feb. 9, 2010, and this application is a continuation-in-part of U.S. patent application Ser. No. 12/272,499 filed Nov. 17, 2008, all of which are incorporated by reference.
1. Field of the Inventions
The present inventions relate to method and apparatus for detecting the locations of shorting defects in a thin film solar cell such as in Group IBIIIAVIA compound thin film solar cells fabricated on flexible foil substrates, and reducing the effect of such defects on the device by cutting a carefully selected section of the solar cell.
2. Description of the Related Art
Solar cells are photovoltaic devices that convert sunlight directly into electrical power. The most common solar cell material is silicon, which is in the form of single or polycrystalline wafers. However, the cost of electricity generated using silicon-based solar cells is higher than the cost of electricity generated by the more traditional methods. Therefore, since early 1970's there has been an effort to reduce cost of solar cells for terrestrial use. One way of reducing the cost of solar cells is to develop low-cost thin film growth techniques that can deposit solar-cell-quality absorber materials on large area substrates and to fabricate these devices using high-throughput, low-cost methods.
Group IBIIIAVIA compound semiconductors comprising some of the Group IB (Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group VIA (O, S, Se, Te, Po) materials or elements of the periodic table are excellent absorber materials for thin film solar cell structures. Especially, compounds of Cu, In, Ga, Se and S which are generally referred to as CIGS-type, or CIGS(S), or Cu(In,Ga)(S,Se), or CuIn1-xGax (SySe1-y)k, where 0≦x≦1, 0≦y≦1 and k is approximately 2, have already been employed in solar cell structures that yielded high conversion efficiencies. Specifically, Cu(In,Ga)Se2 or CIGS absorbers have been used to demonstrate 19.9% efficient solar cells. In summary, compounds containing: i) Cu from Group IB, ii) at least one of In, Ga, and Al from Group IIIA, and iii) at least one of S, Se, and Te from Group VIA, are of great interest for solar cell applications.
The structure of a conventional Group IBIIIAVIA compound photovoltaic cell such as a Cu(In,Ga,Al)(S,Se,Te)2 thin film solar cell is shown in
The conversion efficiency of a thin film solar cell depends on many fundamental factors, such as the bandgap value and electronic and optical quality of the absorber layer, the quality of the window layer, the quality of the rectifying junction, etc. A common practical problem associated with manufacturing thin film devices, however, is the inadvertent introduction of defects into the device structure. Since the total thickness of the electrically active layers of thin film solar cells is in the range of 0.5-5 micrometers, these devices are highly sensitive to defectivity. Even the micron size defects may influence their illuminated I-V characteristics. There may be different types of defects in thin film solar cell structures. Some of these defects may be only morphological in nature and they may be “nuisance defects”, which are not electrically active. Other defects, on the other hand, may be electrically active and may negatively impact the performance of the device. Such defects are sometimes called “killer defects”. Shunting defects, for example, may introduce a shunting path through which the electrical current of the device may leak through and therefore they may be considered to be “killer defects”. Such shunting defects lower the fill factor, the voltage and the conversion efficiency of the solar cells, and therefore, they need to be minimized, eliminated or passivated. Detection and passivation of harmful defects improve the yield of thin film solar cell processing and therefore may be critical for low cost, high efficiency thin film solar cell manufacturing.
Prior work in eliminating shunting defects in solar cells includes work by Nostrand et al. (U.S. Pat. No. 4,166,918) who used an approach to bias the cell and heat up the shunts that carry a high current. A cermet material was incorporated into the cell stack which preferentially formed insulators at the shunt positions during the bias due to local heating. Izu et al. (U.S. Pat. No. 4,451,970) scanned the surface of the solar cell with a contacting liquid head which electrochemically etched or anodized the shorting regions. The etched regions could then be filled with a dielectric. This technique may be applicable for the amorphous Si type solar cells. However, etching or anodizing of CIGS type compound materials leave behind conductive residues comprising metallic species of Cu, In or Ga at the etched location that actually may make shorting even worse than before etching. Phillips et al. (U.S. Pat. No. 4,640,002) used Laser Beam Induced Current (LBIC) technique to locate shorting defects on solar cell structures and then burned the shorts out by using a high power laser beam. A similar approach is recently used in US Patent Application 2007/0227586. Hjalmar et al. (U.S. Pat. No. 6,750,662) scanned the surface of Si solar cells with a voltage point probe and applied a voltage or light bias (illumination) detecting areas with shunts. This approach may work for thick crystalline solar cells, but would damage thin film devices. Glenn et al. (U.S. Pat. No. 6,225,640) used electroluminescence imaging on completed solar cells and removed detected defects chemically. Again such an approach is not applicable to flexible thin film devices such as CIGS cells, because as will be discussed later, defects in such thin film structures need to be detected and fixed before the solar cell is actually completed. Zapalac (US Patent Publication 2007/0227586) used laser scanning, spectroscopic ellipsometry and photoluminescence to determine shunts on finished solar cells and described ways of shunt removal by ablation or scribing.
As the brief review above shows, importance of detection and removal of shunt defects in solar cells has been recognized for many years. Much of the work has concentrated on standard Si solar cells and techniques have been developed to detect shunts in finished devices. In thin film structures using CdTe, polycrystalline Si, amorphous Si, and CIGS absorber layers, the nature and chemical composition of the layers within the device structure are widely different, changing from a single element (Si), to more complex compounds such as a binary compound (CdTe), a ternary compound (ClS), a quaternary compound (ClGS), and a pentenary compound (ClGSS). Therefore, one defect removal method which may work for one device may not work for the other. The laser ablation method that is used for shunt removal, for example, is very successful for Si devices because Si can be easily ablated without leaving behind debris that would affect device performance. For CdTe, process window for laser ablation is narrow because there is the possibility of formation of conductive debris comprising metallic Cd and/or Te at the location where laser ablation is performed. For CIGS, there is to date no successful laser ablation process because laser heating of this compound semiconductor leaves behind conductive phases comprising Cu, In, Ga metals and/or Cu—Se phases. Such conductive phases introduce further shunts in the device structure at the laser ablated locations. Similarly, techniques using chemical etching of defect areas introduce problems for devices employing compound semiconductors such as CdTe and CIGS(S). In such compounds, chemical etching does not etch the material uniformly and leaves behind conductive residue.
For example, an exemplary CIGS type solar cell may have a 100-300 nm thick transparent conductive layer, 50-100 nm thick buffer layer, 1000-2000 nm thick absorber layer and 200-500 nm thick contact layer. The substrate is typically 25-100 μm thick and the grid pattern has a thickness in the 5-50 μm range. In such thin and flexible device structures scribing over the defect with a mechanical tool peels off and damages the various device layers at the vicinity of the defect which already has a shorting path for the electrical current, and also damages the metallic substrate which is flexible and pliable. Such damage from the conducting parts of the solar cell may create conductive debris shorting the top surface of the device to the substrate, the debris originating from the damaged substrate region, the contact layer as well as the transparent conductive layer and especially the portion of the grid pattern damaged by the scribing tool. Laser approaches used to remove defects from standard solar cells also do not work well for foil based thin film devices such as CIGS type devices. First of all, adjustment of the laser power to remove only the top transparent conductive layer or the grid pattern at the defect region is very difficult, and sometimes impossible. The laser beam, by heating the grid pattern and/or the metallic substrate, may cause local melting of the metal substrate and cause new shorting defects, especially for thin film solar cell structures constructed on conductive substrates. Laser removal of CIGS itself may create conductive debris around the removal area comprising metallic species such as Cu, In, and Ga. Such conductive debris is a source of new shorting defects in the device structure. Especially the most serious shorting defects which are under the grid pattern may not be removed by laser processes.
Therefore, there is a need to develop defect detection and passivation approaches that are specifically suited for CIGS-type thin film device structures manufactured in a roll-to-roll manner.
The present inventions relate to method and apparatus for detecting efficiency reducing defects in a thin film solar cell such as a Group IBIIIAVIA compound thin film solar cell, and cutting a section of the solar cell to improve its efficiency.
The present invention provides methods of manufacturing a high efficiency solar cell. In one embodiment, in a solar cell having a grid pattern that channels current, a defect causes an undesired current flow is removed by mechanically removing a portion of the grid pattern, thereby passivating the defect by removing a segment of the solar cell adjacent the defect. The segment also includes the front and back portions of the solar cell at the location of the defect without including the defect.
In different embodiments, different types of input signals can be input and corresponding output signals can be detected. For example, the input signal can induce one of infra-red (IR) radiation, photoluminescence and electroluminescence as the output signal.
These and other aspects and features of the present invention will become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures, wherein:
The embodiments of the present inventions provide defect detection processes, apparatus to detect defects in solar cell structures and methods and apparatus to passivate that defect. In some embodiments, instead of passivating or removing the defect itself, its effect is reduced or eliminated by partially cutting the solar cell structure including the substrate, in predetermined locations. These methods are especially suited for flexible thin film solar cell structures built on flexible thin film substrates such as polymeric substrates and metallic substrates. Such polymeric substrates include, but are not limited to polyimide-based high temperature substrates, and the metallic substrates include but are not limited to stainless steel, titanium, molybdenum, and aluminum containing conductive substrates. The semiconductor absorber layers of such flexible substrates may include CIGS type materials, cadmium telluride type Group II-VI materials, amorphous Si, polycrystalline or microcrystalline Si, organic semiconductors, and absorber layers employed in dye-cells such as dye-titanium oxide containing layers.
In one embodiment a roll-to-roll defect detection and passivation apparatus may be used to detect and passivate the defects formed within a flexible continuous workpiece including a stack of a base, a CIGS absorber formed on the base and a transparent conductive layer formed on the CIGS absorber layer. During the process, initially a section of the continuous flexible workpiece is made substantially flat and an input signal from a signal source is applied to a front surface of the section. The front surface may be the top surface of the transparent conductive layer or a surface of a temporary layer coated on the transparent conductive layer. In response to the input signal, an output signal is generated from a predetermined area of the front surface and detected by a defect detector. The output signal carrying the defect position information is transmitted to a computer and registered in a database. With the position information, an injector device is driven to the defect location to apply an insulator material, preferably an insulating ink, to passivate the defect. If the surface has a temporary layer, the passivation process is performed after removing the temporary layer. A grid pattern layer may be formed over the predetermined area after completing the defect detection and passivation processes.
Certain aspects of the embodiments will now be described using a solar cell structure 30 shown in
As shown in
The exemplary defect regions 27A, 27B and 27C of
If the exemplary defect region 27A described above was located at the location E, although it is away from the busbar 42 and between the two fingers 43A and 43B, the solar cell performance could be affected. As explained before, there is a conductive path between the TCO film 26 and the contact layer 23 at the defect region 27A. However, this shunting path can affect a small area of the solar cell 40 in the near vicinity of the location E. This is because the sheet resistance of the TCO film 26 is relatively high and the current collected by the TCO layer 26 between the finger 43A and finger 43B mostly chooses to flow towards these two fingers and the busbar 42, which have much lower resistance than the TCO film 26. Therefore, the defect region 27A at the location E may affect the solar cell performance, but it does not totally short circuit the device.
If the defect region 27A was at the exemplary location F which is next to finger 43C, its influence on the cell performance would be worse compared to the case discussed above, because the resistance between the finger 43C and the defect region 27A is much smaller due to the shorter distance between them. If the defect region 27A was at the location G or the location H, which are under the finger 43D and the busbar 42, respectively, it would greatly influence the performance of the solar cell 40. In this case, the very low resistance finger 43D and busbar 42 are directly over the defect region 27A, and thus the current collected by the finger 43D and the busbar 42 has a direct low resistance path to the contact layer 23. This is shunting and it is expected to reduce the fill factor, voltage and the conversion efficiency of the solar cell 40 greatly.
If the defect region 27B described above is at the exemplary location E, which is away from the busbar 42 and between the two fingers 43A and 43B, it may affect the solar cell performance. As explained before, there is a conductive path between the TCO film 26 and the contact layer 23 at the defect region 27B. However, this shunting path can affect only a small area of the solar cell 40 in the near vicinity of the location E. This is because the sheet resistance of the TCO film 26 is relatively high and the current collected by the TCO layer 26 between the finger 43A and finger 43B chooses to flow towards these two fingers and the busbar 42 which have much lower resistance than the TCO film 26. Therefore, the defect region 27B at the location E may affect solar cell performance, but it does not totally short circuit the device. If the defect region 27B was at the location F, which is next to the finger 43C, its influence on the cell performance could be worse compared to the case just discussed, because the resistance between the finger 43C and the defect region 27B is much smaller due to the shorter distance between them. If the defect region 27B was at the location G or the location H, which are under the finger 43D and the busbar 42, respectively, it would greatly influence the performance of the solar cell 40. In this case, the very low resistance finger 43D and busbar 42 are directly over the defect region 27B, and thus the current collected by the finger 43D and the busbar 42 has a direct low resistance path to the contact layer 23. This is shunting and it is expected to reduce the fill factor, voltage and the conversion efficiency of the solar cell 40.
The defect region 27C described above may be at the location E, which is away from the busbar 42 and between the two fingers 43A and 43B. In this case, as explained before, there is not a conductive path between the TCO film 26 and the contact layer 23 at the defect region 27C. Therefore, the void 32 at the defect region 27C does not generate photocurrent, but it does not introduce any shunting either. Since the size of the void 32 is typically much smaller than the total area of the solar cell 40, the loss of photocurrent is usually insignificant. For example, for a 100 cm2 area solar cell, a 100 micrometer diameter void introduces only 0.00008% reduction in the generated photocurrent and it does not introduce any shunt at the location E. The situation does not change even if the defect region 27C was at the location F which is next to the finger 43C, i.e. the void 32 at the defect region 27C does not generate photocurrent, but it does not introduce any shunting either. If the defect region 27C was at the location G or at the location H, which are under the finger 43D and the busbar 42, respectively, the situation changes drastically. In this case the defect region 27C may greatly influence the performance of the solar cell 40. Specifically, as the busbar 42 and the finger 43D are deposited over the TCO film 26, the conductive materials such as Ag-filled inks or pastes constituting the grid pattern material, flow into the void 32 and establish a highly conductive shorting path between the grid pattern 41 and the contact layer 23 at the locations G and H. Thus the current collected by the grid pattern 41 find a direct low resistance path to the contact layer 23. This is shunting and it is expected to reduce the fill factor, voltage and the conversion efficiency of the solar cell 40.
As the discussion above indicates, unlike in thick Si solar cells, defects in a thin film solar cell structure negatively impact the solar cell performance, especially when any section of a grid pattern is formed on such defects. Therefore, detection and passivation of such defects in the solar cell structure are needed. Such detection and passivation preferably need to be carried out before the grid pattern is deposited or formed over the cell structure, i.e. before the solar cell is fully fabricated.
In an embodiment of the present invention, a solar cell structure is first fabricated without a grid pattern. Before the grid pattern is formed on the cell structure, a defect detection process is carried out. This process identifies the locations of the defects in the solar cell structure which may or may not be shunts, but would create shunts after finger pattern deposition. A defect passivation process may then be carried out to passivate at least some of the defects that are detected. A grid pattern is then formed on the window layer.
The information about the location of the shunt defects may be saved by a computer and this location information may later be used to passivate the defective regions after the temporary conductive blanket 50 is removed from the top surface of the TCO film 26. It should be noted that the exemplary temporary conductive blanket 50 of
It is also possible to use a combination of different types of temporary conductive blankets. In
After collecting the defect location information, passivation of the defect regions 27A, 27B and 27C may be achieved by forming high resistance caps 60 over them as shown in
Defect detection and passivation may be carried out on individually cut solar cell structures or they may preferably be carried out in a roll-to-roll manner on a continuous solar cell structure.
It should be noted that since the solar cell parameters are most negatively impacted by the presence of defects directly under or in close proximity of the grid pattern, defect detection and defect passivation processes may be limited only to these areas. Doing so increases the throughput of the detection and passivation processes compared to the case that carries out such detection and passivation processes over substantially the whole surface of a thin film solar cell structure. In the high throughput process, the location of the grid pattern to be deposited is predetermined and therefore the defect detection and passivation processes are applied to this predetermined location. Considering the fact that grid pattern in a typical solar cell covers only less than 10% of its total area, this approach reduces the area of defect detection and passivation by 90% and increases the throughput of these processes greatly. For example, the roll to roll processing system 70 may be used in this mode to increase its throughput by 10 times or even more. With the increased throughput it becomes feasible to integrate the roll-to-roll defect detection and passivation process with a high speed roll-to-roll finger pattern screen printing process, i.e. carrying out the screen printing step right after the detect passivation step within the same process tool.
The defect region detection methods may be non-contact or contact methods. In non-contact methods, no electrical contact needs to be made to the solar cell structure such as the solar cell structure depicted in
A roll-to-roll defect detection and passivation system 90 is shown in
During the defect detection process an input signal is applied from an input signal source to the predetermined detection area, and an output signal from the predetermined area carrying the defect information is collected by the detector. Exemplary, input signals may be delivered through shining light onto the detection area, applying a voltage between the detection area 95 and the substrate of the workpiece. The output signals may be infra-red (IR) radiation, photoluminescence radiation and electroluminescence radiation and the like, each of which can be operated in DC or AC “lock-in” mode. As sections of the workpiece 94 is advanced from the supply spool 91 to the receiving spool 92 either in a continuous motion or in a step-wise motion, preferably, in a step-wise motion, defect detection is carried out in the detection area 95 and the position information of the detected defects carried by the output signal are recorded by a computer (not shown). This position information is then used to passivate the detected defects in a passivation location. When the detection area 95 moved from the detection location to the passivation location of the system by moving the workpiece 94, it becomes passivation area 100. In essence, both the detection area and the passivation area are the same predetermined area on the front surface of the section of the continuous workpiece in two different locations where the detection and passivation processes can be applied. An injector 99 which may be moved across the front surface 96A of the workpiece 94 over the passivation area 100 goes to the position where the defect has been detected and dispenses a high resistivity ink over the defect region. If the ink is heat-cured or UV-cured type, such curing means may also be applied to the deposited ink (not shown). It should be noted that the detection area 95 and the passivation area 100 may actually be the same area, i.e. both defect detection and passivation may be carried out one after another when the workpiece 94 is kept stationary. However, separating the detection and passivation steps increases throughput. In fact, for higher throughput there may be two or more detection areas with two or more detectors and two or more passivation areas with two or more injectors.
As mentioned before, it is also possible to integrate defect detection and passivation process with a finger pattern deposition step. In this case, the sections of the continuous workpiece that received defect detection and passivation steps may move to a screen printing/curing unit (not shown) of the overall system before the sections are advanced to the receiving spool.
It should be noted that the nature of the detector 98 depends on the defect detection method and the input signal source employed. The embodiments of the present invention may use both non-contact detection methods and contact detection methods. The non-contact defect detection methods may utilize techniques such as photoluminescence and infrared thermography.
Another non-contact approach is photoluminescence process. In this case light is shone through the opening 102 of the mask 103 as the input signal and the detector 98 detects photoluminescence coming from the solar cell structure as the output signal. In this case all non-active regions, i.e., regions that do not generate photoluminescence, in the solar cell structure including shorted regions, open circuit regions and regions comprising foreign particles, etc. may be detected. As mentioned before, these techniques can be operated either in a DC or AC mode. In the latter case the excitation source or input source is modulated, and the images are acquired and processed to as to “lock-in” on the signal of interest, with a substantially improved signal to noise ratio. For the specific CIGS solar cell structure the non-contact detection method using photoluminescence is attractive because it avoids surface damage and can detect defects that are shorts as well as defects that may not be shorts but would create shorts when a finger pattern is deposited. It should be noted that the mask 103 may be in the shape of the predetermined grid region 81 of
For contact detection methods, electrical contacts need to be made to the top and bottom electrodes of the solar cell structure to provide an input signal. An example is shown in
Two exemplary detection methods, thermography and electroluminescence may use the conductive rollers to generate input signals. In thermography, a voltage is applied between the rollers and a bottom contact to the back surface 96B of workpiece 94. Short circuiting defect regions in the detection area 95 pass higher current than the rest of the device and therefore get hotter. An IR camera is used as the detector detects the IR radiation from the hot spots as the output signal. As described above, this technique can detect short circuits effectively. However, defects that do not pass high current in the solar cell structure may go undetected. The second exemplary contact method is electroluminescence where a voltage or input signal is applied between the rollers and a bottom contact to the back surface 96B of the workpiece 94 and an electroluminescence detector senses the radiation or the output signal coming from the detection area 95. Shorted areas, open circuited areas as well as areas containing foreign matter may be detected this way as dark, inactive regions of the cell structure. Therefore, electroluminescence detection is preferred for CIGS solar cell structures since it can detect defects that may not be shorts but would create shorts when a finger pattern is deposited.
In another embodiment of the present invention, the above described defect detection methods and techniques may be applied to a completed solar cell structure including a terminal, such as a grid pattern, on the front light receiving surface of the solar cell to detect efficiency reducing defects such as shunting defects. In context of this application, an efficiency reducing defect is the defect that electrically shorts or short circuits the back contact or the conductive substrate of the solar cell to a location which includes a particular portion of the grid pattern such that electrons (or electrical current) collected by the grid pattern leaks into that defect from that location and communicated to the back contact or conductive substrate. Such a leaky defect may be due to an ohmic shunt or it may also be due to a leaky diode at the location, i.e. the leakage current may be due to a weak or leaky diode characteristics at the location. The defect may be under or in very close proximity of the location so that it pairs itself electrically with this location by switching the direction of electron or current flow from the busbars to the defect, resulting in electron or current loss. Such locations surrounding the defect while including a portion of a grid pattern will be referred to as paired location hereinafter, i.e., the location that is electrically connected to the defect by delivering electrons or electrical current to the defect from that location. In this embodiment, after an efficiency reducing defect is detected by one of the methods described above, the negative effect of this defect on the cell is reduced by reducing the size of the paired location of this particular defect by a passivation process of the present invention without removing the defect from the solar cell. The passivation process generally involves reducing the size of the paired location by physically and electrically isolating the defect from the entire grid pattern or at least a portion of the grid pattern by cutting through the grid pattern and the whole device structure at locations close to the defect so as to inhibit unwanted electron or current flow into the defect from the parts of the grid pattern beyond the cuts made. As will be described more fully below, this may be done by forming a gap between the defect and the grid pattern, for example, by removing a segment of the solar cell including some grid pattern material if the defect is in proximity of the grid pattern. If the defect is in contact with a particular portion of the grid pattern, this particular portion is also physically and electrically separated from the rest of the grid pattern as the segment of the solar cell is removed. The material removal process may be physical cutting and it may be conducted in a cleaning solution to remove small dust or material pieces resulting from the removal process. The material removal process may be further conducted in a mildly acidic solution to dissolve small particles resulting from the process.
There are two exemplary defects shown in
It should be noted that the lengths of the first paired location 126A and the second paired location 126B shown in
As shown in
The completed solar cell of the top portion 120 shown in
As mentioned above, the first defect 124 and the second defect 125 reduce the performance of the completed solar cell. The shunt resistance introduced by such defects reduces the fill factor and thus the efficiency of the device. As discussed before, some prior art methods used chemical approaches to etch away or anodize at least one of the transparent conductive layer, the absorber layer or the contact layer of the solar cell at the exact location of the defect with the goal of passivating the defect. In another approach a laser was used to ablate the defective region. Yet another technique applied a physical tool such as a scriber on the defect with the goal of physically eliminating it. Such approaches do not yield good results for thin film solar cell structures, especially for devices employing compound semiconductor absorber layers constructed on conductive foil substrates. For example, a CIGS-type solar cell fabricated on a 25-100 μm thick metallic foil substrate, chemical etching or anodization methods do not work well because CIGS is made of Cu, In, Ga and Se and chemical or electrochemical etching of this compound material does not remove all these different materials at the same rate and leaves behind residues that may be conductive. Therefore, while removing a shorting defect, new shorts may be introduced in the device structure where the chemical or electrochemical etching process is performed. Furthermore, if the defect is under the grid pattern, etching techniques cannot be used because etching the grid, which is a thick layer, is not very practical. Mechanical processes that try to scratch away the defect may introduce even more defects, especially since the defect itself comprises highly conductive debris shorting the device. Physical scratching right on the defect may actually smear such conductive debris and often make the electrical shorting even worse in thin film solar cell structures.
In the following part of the description, a method of reducing or eliminating the influence of defects without disturbing the defects or removing the defects from the solar cell will be described in connection to
Similarly, the second defect 125 is electrically separated from the busbars 123A and 123B by forming a second gap 132B and a third gap 132C on the finger 122C. The second defect 125 is located between two busbars 123A and 123B or current collectors. Therefore, the size of the paired location 126B can be substantially reduced by reducing the distance between the second gap 132B and the third gap 132C on two sides of the second defect 125. In this configuration, due to the second gap 132B, electrons in a first remaining section 136A of the finger 122C flow toward the busbar 123A; and due to the third gap 132C, electrons in a second remaining section 136B flow toward the busbar 123B. A second sacrificed portion 135 of the finger 122C, which is the upper portion of the second paired location 126B, is electrically isolated by forming the second and third gaps 132B and 132C. The second defect 125 is physically and electrically separated from the body of the grid pattern 122, and the electron flow into the second defect 125 is limited with the size of the second sacrificed portion 135 of the second paired location 126B. Therefore in order to keep a larger portion of the finger 122C active and contributing to the device efficiency, the second and third gaps are located close to the second defect 125. A preferred distance from the defects to the cuts may be in the range of 0.5-2 mm. It should be noted that the preferred method of forming the gaps 132A, 132B and 132C is removing the whole solar cell structures at these locations including the substrate. Therefore, the technique applies to thin film solar cells employing flexible substrates. Such removal can be achieved using tools such as cutting wheels, die cutters, stamping tools, etc., that cut through the whole solar cell structure including the front portion and the back portion, which includes the substrate.
The passivation process of the present invention may be used very efficiently with solar cells using a mesh grid pattern 200 comprising a network of fingers 202 formed on a solar cell surface 204. Current collected by the network of fingers 202, which is depicted by arrows, flows to a terminal ‘T’ which is electrically connected to the network of fingers. Each defect 206 is passivated by forming two gaps or cuts, namely a first gap 208A and a second gap 208B to reduce the size of its respective paired location 210 as described above. As can be seen in
In a process sequence the locations of the defects are detected in a defect detection station and the passivation process is applied in a passivation station where the gaps may be formed using a cutting apparatus. The present invention may be carried out in batch mode involving the defect detection and position determination, and the passivation processes being carried out in batch mode on individual finished solar cells. Alternately, the defect detection and passivation steps may be carried out in a roll-to-roll manner before the solar cells are individually cut from a flexible workpiece. Furthermore, it is also possible that the defect detection and location determination may be carried out in a roll-to-roll manner and then the passivation process may be applied to the individual solar cells in batch mode after the individual solar cells are cut out of the roll of a continuous solar cell structure and separated from each other. The advantage of this last approach is the fact that the passivation step and the tool used for that operation may be simple to build and operate.
Although the present inventions are described with respect to certain preferred embodiments, modifications thereto will be apparent to those skilled in the art.
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|U.S. Classification||29/402.06, 29/825, 29/402.03, 349/73, 29/832, 29/846|
|Cooperative Classification||Y02P70/521, Y10T29/49721, Y10T29/49726, Y10T29/4913, Y10T29/49155, Y10T29/49117, H01L31/186, H02S50/10, H01L31/046, H01L31/0749, H01L31/03928, G01N21/9501, G01R31/025, Y02E10/541, H01L31/18|
|European Classification||H01L31/18, H01L31/18G, H01L31/0749, H01L27/142R2, H01L31/0392E2|
|May 4, 2010||AS||Assignment|
Owner name: SOLOPOWER, INC., CALIFORNIA
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:BASOL, BULENT M.;REEL/FRAME:024333/0921
Effective date: 20100305
|Aug 9, 2013||AS||Assignment|
Owner name: SPOWER, LLC, OREGON
Free format text: MERGER;ASSIGNOR:SOLOPOWER, INC.;REEL/FRAME:030982/0818
Effective date: 20130730
|Aug 13, 2013||AS||Assignment|
Owner name: SOLOPOWER SYSTEMS, INC., OREGON
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:SPOWER, LLC;REEL/FRAME:031003/0067
Effective date: 20130809
|Feb 27, 2015||REMI||Maintenance fee reminder mailed|
|Jul 19, 2015||LAPS||Lapse for failure to pay maintenance fees|
|Sep 8, 2015||FP||Expired due to failure to pay maintenance fee|
Effective date: 20150719